1. Technical Field
The present invention relates to the upper internals of nuclear reactors and it is particularly suitable for use in water cooled and moderated reactors.
2. Prior Art
Such reactors are already known having a core formed by adjacent upstanding fuel assemblies, and internals comprising a lower plate formed with outlet openings for the coolant flowing out of the fuel assemblies, an upper plate, supporting columns connecting the plates together and tubular guides for control clusters movable vertically between a position in which they are inserted in the core and a position in which they are out of the core. The guides are individually fixed to the upper plate and each provided with a device for guiding and centering their lower part on the lower plate.
The purpose of the control cluster guides is to guide the rods of the clusters, to prevent deformation thereof and to protect these elements whatever their position during normal operation of the reactor and under accident conditions. They generally comprise a tube, transverse plates, frequently called "cards" distributed along the upper part for individually guiding the rods at intervals and, at its lower part, split tubes each assigned to a rod. Such guides are described for example in U.S. Pat. No. 4,173,513 (OBERMEYER) et al), French Pat. Nos. 83 09275 (FRAMATOME), and 81 05037 (FRAMATOME).
Various devices for guiding and centering the guides with respect to the lower plate of the internals are known. They are generally so designed that the guides can move axially with respect to the lower plate, particularly to allow free expansion thereof during use and for facilitating mounting or removal as required, while closing the clearances necessary for slidably guiding the rods, for avoiding vibration of the guides induced by the turbulence generated by the coolant. The devices must limit lateral deflection of the guides in the case of an earthquake or accidental depressurization of the reactor, for instance upon failure of the outlet nozzle, so that the guides remain alignmed with respect to the fuel assemblies and continue to allow free movement of the control clusters. The guide devices are often formed of resilient members which have the disadvantage of simultaneously fullfilling all the preceding functions. The design parameters are consequently critical since the guiding devices must both be sufficiently flexible for accomodating assembly misalignments resulting from the manufacturing tolerances of the guides and the inner and upper plates of the upper internals and sufficiently rigid for preventing vibration of the guides and deflection thereof in the case of accidental stresses.
Such prior art is examplified by centering devices formed by a split pin, engaging in a hole in the lower plate (U.S. Pat. No. 4,173,513). These known devices require accurate positioning so that engagement of the resilient members into the holes provides in the lower plate does not generate excessive stresses in the resilient members.